Multi-frequency antenna



Feb. 20, 1962 J. M. BOY-ER MULTI-FREQUENCY ANTENNA 3 Sheets-Sheet 1 Filed Oct. 29. 1953 Feb. 20, 1962 .1. M. BOYER MULTI-FREQUENCY ANTENNA 3 Sheets-Sheet 2 Filed Oct. 29. 1953 NE a:

M73 2 E m as NR Tn N anus United States Patent 3,022,507 lvlULTI-FREQUENCY ANTENNA Joseph M. Boyer, Redondo Beach, Calif., assignor to Antenna Engineering Laboratories, Torrance, aiif., a limited co-partnership Filed Oct. 29, 1953, Ser. No. 388,981 6 Ciaims. (Cl. 343-818) This invention relates to antennas for radio wave sending and reception and particularly to an antenna construction for use in the ranges commonly known as short wave, very high, ultra high, and micro wave frequencies.

While in the following specification, the invention will be described as applied to the so-called short wave range (1 to 30 megacycles) it will be evident to those skilled in the art that the disclosed principles of construction and operation can be readily adapted in the light of such disclosure to other ranges of wave lengths.

By way of example, within the short wave range, the amateur radio operators have been assigned certain short wave bands by international treaty; said bands being as follows:

vWave length, meters: Frequency megacycles In the use ,of these bands, the need is for antenna means which may be used without switching from one band to another in order to take advantage of, for example, changing ionospheric propagation conditions so as to employ that band which at the moment is found to be most suitable to maintain reliable communication conditions.

The usual sending and receiving equipment other than the antenna employed by amateur operators is designed to operate efliciently on all of these bands, but usually in order to operate efhciently on all of these bands the operator requires separate antennas for each band and lacking either space or the means for such equipment, must satisfy himself with a compromise antenna with the attendant additional tuning and impedance matching means to enable him to change from hand to band Generally with such equipment, only one band can be employed with eificiency which is markedly greater than the other bands; and, since the antenna is a compromise, even that band is not to be operated at an eificiency which is equal to the maximum performance to be realized when the antenna is perfectly matched to a single wave length. These same considerations apply as well to all other situations in which it is desired to send and receive signals selectively on a plurality of discrete wave lengths with a minimum of antenna equipment and with maximum efliciency and convenience on each of the wave lengths involved.

The present state of the antenna art includes antennas of large cross section in terms of Wave length which may be used over wide frequency bands while maintaining substantially constant input impedance with resultant good impedance match to the transmission line connected to the antenna terminal, but such antennas are practical only on the very high, ultra high, and micro wave frequency bands but are impractical for frequency bands between 1 and 30 megacycles due to the fact that the resulting structures would be so large as to be prohibitive both from size and cost standpoints.

Having the foregoing considerations in mind, the principal object of the invention is to provide a single antenna which will permit the transmission and reception of radio signals on at least two or more widely separated frequency bands while maintaining at its input terminal 3,022,567 Patented Feb. 20., 1962 2 a substantially constant input impedance on each of the frequency bands for which it is designed.

Another object of the invention is to provide an antenna construction embodying the said principal objective which is simple, of pleasing appearance, which is suficiently rugged to withstand adverse weather and other conditions, and which may be installed and erected in limited space areas.

A further object of the invention is to provide a simple, multiple frequency band antenna which may be employed to excite a non-resonant reflecting structure such as a parabolic reflector to effect highly directive radiation pat terns on several frequency bands.

Still another object of the invention is to provide an antenna element which is efiective to utilize single frequency directive resonant array structures such as the Yagi parasitic array, or combinations of driven array configurations, to obtain efiicient, directive radiation patterns therefrom on several frequency bands.

With the foregoing objects in view, together with such additional objects and advantages as may subsequently appear, the invention resides in the parts, and in the construction, combination and arrangement of parts described, by way of example, in the following specification of certain modes of execution of the invention; reference being had to the accompanying drawings which form a part of said specification and in which drawings: FIG. 1 is a side elevation of an antenna structure embodying the present invention; portions being broken away to disclose interior construction and to reduce the illustrated length of the antenna,

FIG. 2 is an enlarged, medial sectional view of the lower end of the antenna shown in FIG. 1,

FIG. 3 is an enlarged, fragmentary, medial sectional view taken on the line 33 of FIG. 1 and showing details of structure at the upper end of the coaxial line portion of the antenna structure,

FIG. 4 is an enlarged, side elevation of the upper end of the coaxial line portion of the antenna showing the addition of a means effective to render the antenna capable of sending and receiving signals on at least one frequency band in addition to the said at least two other frequency bands which may be transmitted or received on the antenna,

FIG. 5 is a medial sectional view taken on the line 55 of FIG. 4, a

FIG. 6 is a transverse sectional View taken on the line 66 of FIG. 4,

FIG. 7 is a graph showing the input terminal resistance component characteristics of an actual test specimen embodying the construction of the invention shown in FIGS.

FIG. 8 is a series of graphs showing the voltage standing wave ratio characteristics of the said test specimen as measured on 50 ohm characteristic impedance coaxial cable over the frequency extent of several bands,

FIG. 9 is a series of graphical representations of the voltage distribution patterns of the said test specimen with relation to the graphs shown in FIGS. 8 and 9,

FIG. 10 is a side elevation, partly in section showing a balanced antenna construction embodying features of the invention,

FIG. 11 shows a directive, resonant, multiple frequency antenna array embodying features of the invention, and

FIG. 12 shows the principles of the embodiment of the invention shown in FIG. 10 combined with a non resonant reflector for directive, multiple frequency operation.

Referring to the form of the invention shown in FIGS. 1, 2 and 3, the illustrated embodiment of the invention comprises an antenna structure 1 including a hollow vertical metal tube 2 supported with its lower end spaced 3 above a metal base Plate 3 by a larger and shorter metal tube 4 mounted on the baseplate 3 and coaxially disposed with respect to the tube 2 through a shorting plug 5 comprising a flat metal ring having its outer perimeter secured to the upper end of the tube 4 and having 'a central opening through which the tube 2 extends and to which the tube is rigidly fixed as by welding as best shown in FIG. 2. The upper end of the tube 2 carries a laterally extending annular flange 6 provided with a series of eye bolts 6 to which =guywires 7 are attached; said guy wires including insulators 8 interposed therein and disposed in such proximity to each other along the wire 7 as will not produce resonance on any of the radio free quency bands on which the antenna operates. At its lower end within the tube 4, the tube 2 carries a ring insulator 9 of good electrical insulating properties and good resistance to compression forces held in place thereon by any suitable means such as the rings 9', 9 welded to the tube 2 above and below the insulator 9 and this insulator closely fits the inner surface of the tube 4 and combines with the shorting plug 5 and the guy wires 7 to hold the antenna structure erect.

Mounted on the plate 3 and extending coaxially through the tube 2 is a cylindrical metal rod 10 which beyond the upper e'nd of the tube terminates in a tapered pointed portion 11. At the upper'end of the tube, the rod 10 is thoroughly insulated from the tube 2 by an insulator 12 and within the tube it is held in coaxial alignment with the interior of the tube by several spaced rings 13 formed of plastic foam material. Such material (depending on its density) has been found to possess a dielectric con stant less than 1.1 with reference to air and it may be as low as 1.01 to 1.03 and these values for practical purposes are thesame as air, and thus while not materially effecting any loss, this material serves to hold the rod 10 centered the tube 2. Any suitable means may be employed to hold the rings 13 in place within the tube 2 such as small flanges14 welded to the rod at each side of each of the rings 13 to hold the rings in spaced relation while the rod and the thus secured rings are assembled into the tube 2. V

In the specific example of the invention above described the following dimensions were embodied as indicated by the corresponding letters applied to FIG. 1:

Symbol Definition- Total height of antenna, feet Base to top of tube 2, feet Base to top of tube 4, feet Diameter of tube 4, inches 4 Diameter of tube 2, inches 2% Diameter of rod 10 within tube 2,inches- 1% Height of rod 10 above tube 2, feet 27 Diameter of rod 10 at insulator 12, inches l Distance of lower end of tube 2 to base plate, 1%

Assuming now that the base plate 3 of the antenna above described which is the feed point is connected by a transmission line L to a radio transmitter R operating at a frequency of 3.8 mc., the radio currents. will travel end of the tube 2, the current finding the lower; end of the tube spaced from the base plate will, in turn, excite currents in the coaxial line formed by the outer surface of the tube 2 and the inner surface of the tube 4 terminating in an electrical short formed by the shorting plug'5'.

2 and 4, the outer surface of rod 1Q within the tube 2 and the inner surface of the shorting plug 5. The electrical length of this folded, shorted, cow'al line is considerably less than a quarter wave length at a frequency of 3.8 mc. and consequently a positive reactance will appear between the upper end of tube 2 and the point at which rod 10 emerges through the insulator 12 to form the tip portion 11 thereof. The current will then flow on the said antenna structure in a pattern having maximum voltage value at the distal end of said tip and dropping to zero at the base plate 3.

Onan antenna of the given dimensions, the said positive reactance formed between the opposite ends of the insulator 12 will appear to currents of 3.8 mc. frequency as an inductor or coil interposed in series between the nearest exterior surface points on the rod 10 and the tube 2 separated by the exterior surface ofthc insulator 12. The magnitude of the said positive reactance will be determined by the electrical length of the said coaxial line and the characteristic impedance thereof and results in a substantial reduction in the overall length of the antenna as compared with the length of a simple vertical pipe radiator for that frequency.

To reduce power loss, it is obvious that in a coaxial transmission line such as here incorporated in a radio signal radiator, it is desirable that the insulation between the inner surfaces shall be air, or at least substantially air with respect to dielectric constant and loss tangent. In a location in which the exposed end 11 of the rod 10 would not be subjected to windpressure or other lateral force with resultant bowing of the portion of this rod between the insulator 12 and the base plate 3, or to shorting as by entrance of moisture as from rain or dew the insulation could be air only, but since such locations are generally non-existent, it is preferred to pro vide the centering rings 13 formed of the said plastic foam; a characteristic material being known as Styrafoam, which rings because of being formed largely of air cells have a low dielectric constant with reference to air and a low loss tangent which is essentially that of air and which having good resistance to compression, provide the means by which the rod is maintained centered within the tube 2. Naturally, the less such insulators are employed, the greater the efiiciency of the antenna, but it has been found that if necessary, the entire cavity between the rod 10 and the interior of the tube 2 can be filled with the said plastic foam without lowering the efficiency of the antenna to a point where it fails to function satisfactorily at a plurality of frequencies. Thus, in calculating an antenna for a given wave length or lengths in which the present invention is incorporated, the presence of the said insulators can be disregarded and the calculations can be based on the conducting elements alone.

Referring next to FIGS. 7, 8 and 9 and to those portions thereof at the left hand side of each of the figures ing action of plug 5, a voltage standing Wave pattern will be noted that the impedance is sufl'iciently close to 50 ohms to make a good match with a standard transmission line of that impedance characteristic, that the voltage standing wave ratio is substantially 1:1 and that the voltage distribution pattern shown in FIG. 9 indicates that the entire length of the antenna operates as a quarter wave radiator; it being noted that in the region of the said positive reactance, the said voltage distribution pattern departs slightly from a simple sine wave form normally observed in most antennas as an incident to that reactance. It is also to be noted that the impedance curve rises extremely sharply beyond a frequencyof, say 5 mc.,

so much so, in fact as to render the antenna unusable Thisexcitation of the currentssets up a voltage standing 7' wave pattern along the entire inside surface of the shorted, folded coaxial line formed by the inner surfaces of tubes m Amie-a earth plane and that the eiiect of the said positive reactance is to reduce the overall height of the antenna from that which would be required for a simple vertical pipe radiator for that wave length. The input of this antenna is approximately 35 to 40 ohms. The radiated signals are omnidirectional in the horizontal plane and in the vertical plane are confined largely to the horizon and a pointapproximately 40 degrees above the horizon; the energy dropping rapidly to zero at higher angles. By the law of reciprocity, it is evident also that this antenna will be equally efiicient in reception of signals within the considered band of 3.5 to 4 mc. On reference to FIG. 8 it will be noted that within this band the antenna operates with a negligible loss of power between the transmission line L and the antenna due to change in base input impedance.

Next, assuming the identical antenna is connected to a transmitter in the band of 77.3 mc., and that for example, the transmitter delivers current to the antenna through transmission line L at 7.15 me. or in the midpoint in that band, the radio frequency currents will flow up the outside surfaces of tube 4, shorting plug 5 and tube 2 to the upper end of tube 2, at which point the inner surface of tube 2 and the outer surface of the rod 10 within tube 2 will be excited and due to the shorting action of plug 5 a voltage standing wave pattern will be set up inside the folded, shorted, coaxial line formed by the said tubes, rod and shorting plug. However, at the frequency now under consideration, the said folded, shorted, coaxial line is electrically a quarter wavelength.

This causes an extremely high resistance to appear between the upper end of tube 2 and the horizontally adjacent point on rod 10. The magnitude of this resist.- ance because of the exceptionally high Q value of the air insulated, shorted, folded, coaxial line, will, for all practical purposes disconnect or decouple the portion 11 of the rod 10 which projects above and beyond the insulator 12 from the rest of the antenna and only the outside surface of the antenna below the insulator 12 will radiate. The radiation pattern is similar to that described in connection with operation at 3.8 me.

When the above described transmitter is excited at a frequency of 21.2 mc., the energy from the transmission line L will set up currents on the outside surfaces of the tube 4, shorting plug 5 and tube 2 up to the insulator 12. At this frequency the voltage will fall to zero at a point one quarter wave-length below the upper end of the tube 2 and reach a maximum value at said upper .end. The previously described decoupling action of the folded coaxial line formed by the inner surface of the tube 2 and the outer surface of the rod 1%) within the tube again takes effect. Now, because the portion 11 of the rod is capable of oscillating independently, by virtue of its electrical length, a 180 phase reversal will take place between the upper end of the tube 2 and the horizontally opposite point of the rod 10. The voltage will fall to a very low value at about the mid point of the rod tip 11 and rise to a maximum at the tip, operating at a high efliciency due to the low loss inner construction which, for all practical purposes, is air. Consequently, the antenna now operates as a vertical, phased, co-linear array as indicated by the voltage distribution curves in FIG. 9. On reference to FIG. 7, it will be noted that the base input impedance is approximately 40 ohms afiording a good match for the transmission line L, and on reference to FIG. 8, it will be noted that the reflection power loss efiect is eflicient and remains less than 3% in the region of 21.0-2l.450 mc.

When the transmitter R delivers energy to the antenna at a frequency of 28.0 to 30 mc., the operation is somewhat difierent than that hereinbefore described. Under these conditions, the inner shorted coaxial line portion of the antenna is computed as one wavelength electrically and as is well known in the art a shorted coaxial line which is electrically one wavelength shows a virtual short t5 circuit across its output terminals; such terminals in this case being the upper end of the tube 2 and the point on the rod 10 horizontally adjacent the upper end of the tube 2. The impedance at this point being extremely low, the entire length of the antenna contributes to the operation as a vertical radiator computed as seven eighths waves in extent. Heretofore, vertical radiators of seven eighths waves have been regarded as high angle radiators, but for some reason not yet factually determined, the present antenna operates efliciently at a low, vertical radiation angle and with a substantial gain over that of a quarter wave vertical antenna operating at that frequency. The base impedance is still in the region of 40 ohms thus being a good impedance match for the transmission line L. The voltage standing wave ratio is still far less than the minimum acceptable ratio of 2:1 as shown in FIG. 8.

Referring next to the modified form of the invention shown in FIGS. 4, 5 and 6, the modification comprises a cage element 15 surrounding the upper end of the tube 2 and comprising an upper metal flange 16 fixed to the under side of the flange 6 by the eyebolts 6" which for this purpose in addition to providing anchorage for the guy wires are provided with longer shanks. A non-conductive flange 17 surrounds the tube 2 at a distance below the flange 16 and is provided with peripheral notches 18 in which the lower ends of a series of vertically disposed metal rods 19 extending from the flange 16 are anchored in spaced relation to each other and in uniformly spaced relation radially from the outer surface of the tube 2 by a strap 19. Except for the lower wind resistance achieved by this construction, a section of metal tubing similarly spaced from the tube 2 might equally well be employed in place of the rods 19; the use of nine or more of the said rods being the electrical equivalent of such section of tubing. The length of the rods is such that the length of the antenna below the flange 17 is electrically one quarter of the wave length of the center frequency of the band to be attained by the use of this attachment and in the illustrated embodiment is for the band of l4l4.4 me. The cage and tube thus form a shorted quarter wave coaxial line with the tube 2 serving as the inner conductor.

Energy derived from the transmitter R travels up the outer surfaces of tubes 2 and 4 and shorting plug 5 to excite currents on the coaxial line formed by that portion of the tube 2 contained Within the cage structure and since this coaxial line is shorted at one end, an extremely high resistance or impedance forms between the lower ends of the rods 19 and the horizontally adjacent outer surface of the tube 2 opposite the rods 19 thereby, as indicated by measurements, preventing excitation of the remainder of the antenna. The portion of the antenna below the flange 17 therefore becomes a vertical quarter wave radiator operating in conjunction with the earth plane on the said band of 1414.4 mc., yielding a base impedance of between 35 to 40 ohms. The presence of this cage has little effect on a frequency of 3.5-4 me. because the length of the coaxial line formed by the cage is only A of a wavelength at that frequency. On the band of 7-7.3 mc., the cage is A; of a wavelength in length and as is known, a shorted coaxial line which is A; of a wavelength in length presents at its input terminals an impedance which is equal to the characteristic impedance of the coaxial line itself. In the present case, the illustrated design, the spacing between the rods 19 and the surface of the tube 2 is such that this impedance is in the neighborhood of 20 ohms and this has little effect from an inductive standpoint on the performance of the antenna when operating on the 7-7.3 rnc. band. It more exact performance is desired, compensation can be made by a slight shortening of the tube 2 in those installations in which this modification is used. On the 21-2145 mc. band, this cage has little effect since with relation to that frequency it is an odd number of eighth waves in length with resultant yielding of the same'low series resistance.

aocaso'r (3n the 283() mc. band, the coaxial line formed by the cage is one half wave long thereby forming a virtual short circuit between the flange 17 and tube 2 and consequently having no appreciable deleterious effect when the antenna is'operating on this band. It is believed to be obvious that the foregoing modification can thus be applied to suit other wave lengths on antennas'for other than the short wave bands so long as the addition thereto does not in any of the manners above discussed interfere with the operation of the antenna to which it is added on the wave length or lengths for which the antenna is designed to operate.

Referring nexttoFlG. 10 there is shown the application of the invention to a balanced, antenna structure comprising two identical antennastructures of the same general form as shown in FIG. 1 disposed with theirbase or input ends adjacent each other and with the two structures located on a common axial line. Since the structures are identical, a description of one will sufiice for both. Such balanced antennas are mounted remote from a conductive plane and may be disposed at any angle relative to such conductive plane.

' Each antenna structure comprises a metal base element ZOhaving a shallow conical'end to the point of which a lead 21 affording connection with a transmis:

sion line L common to both structures. Mounted on the base element and projectingaway from the pointed end of the base element is a metal tube 22 corresponding in location and function to the tube 4 in the first described form of the invention; said tube terminating at its distal end in a metal disk 23 serving both as a shorting plug and, as a mount for a second tube 24 mounted in the disk and extending in coaxial relation to the tube 22 to an inner end 25 spaced from the inner face of the base element 20 and further supported in said coaxial relation by an insulator ring 26 surrounding the tube 24 and en gaging the inner surface of the tube 22 in the same manner as the insulator 9 in the first described form of the lnve ntion.

The tube 24 projects beyond the shorting plug 23 and at its distal end carries an insulating plug 27 serving as an outer support for a metal rod 28 disposed coaxially within the tube and having one end conductively connected to the base element at 29 and the other or distal end disposed beyond the distal end of the tube 24. The insulator plug 27 in addition to supporting the rod 28, also servesrto seal the end of the tube against the entrance of moisture which might result in unwanted loss in the. operation of the antenna. While air insulation between the interior parts of the coaxial construction is the most desirable, in those antennas embodying the last described construction, the use of one or more insulators between the rod 28 and the interior of the tube 24 to 'm'aintain the desired coaxial relationship as indicated at 31} can be employed if such insulators are of a character having a dielectric constant sufiiciently close to that of air' as previously discussed. In those antennas of the last described type, adapted for the longer wave lengths for which horizontal antennas may be conveniently employed without structural support complications, such insulators may be necessary.

The mode offoperation is believed to be obvious from the detailed description of the operation of the first described form of the invention with the additional application of known principles of the antenna art to opposed horizontal antenna constructions. The overall electrical length of each of the two units is one-quarter of the wavelength of the longest frequency for which the antenna is designed and the electrical length of each unit from the base thereof to the distal end of the tube 24 is onequarter of the next highest frequency at which the antenna is to be resonant. Additionally, such balanced antennas will operate in, all respects on other frequencies inv the same manners as the previously described vertical embodiment. Further, the combination of elements hereinbefore described can be applied to any parasitic or driven arrayconfiguration with the same capability of' multiple frequency operation in those configurations that obtains in the first described embodiment of the invention except as restricted by the physical laws governing the performance of arrays at separately considered singlefrequencies.

'By way of example, referring to the embodiment of the invention shown in FIG. 11, the antenna disclosed in FIG. 10 is shown employed as the driven element in a so-called parasitic or .Yagi array. Since the driven element is identical with that, shown in FIG. 10' the same numerals are applied thereto and repetition of the structure is omitted. At one side of the driven element is a reflector element 31 comprising a first metal sleeve 32 of the same diameter as the sleeves 22 of the units comprising the driven element; said sleeve being longer by approximately 5% than the distance from the distal end of one of the sleeves 22 to the distal end of the other sleeve. 22. 'At its mid length, the sleeve 32 carries a disc like base member 33 and at each end it is provided with a shorting plug element 34 corresponding to the shorting plugs 23 in thc driven element; each of said shorting plugs having a coaxially disposed opening therethrough in which a second tube 35 is secured. Within the tube 32 the tubes 35 extend to inner ends 36 spaced from the adjacent faces of the base member 33; said tubes being held in coaxial alignment by insulators 37 and at their other ends the tubes 35 project beyond the shorting plugs to equal extents to an overall length that is likewise about 5% greater than the overall length be-.

tween the distal ends of the tubes 24 in the driven element. Mounted in the distal ends of each of, the tubes 35 is an insulator 38 having a. coaxially disposed opening extending therethrough in which the mid portion of a metal rod 35' is mounted; said rods at one end thereof extending coaxially through the tubes 35 and being secured to the base members 33. The distal ends of the rods 39 terminate at equal distances beyond the insulators 38 and terminate at an overall dimension which is about 5% greater than the overall distance between the distal ends of the rods 28 of the driven element. The electri cal length of the folded coaxial lines formed by the tubes 31 and 35 and the shorting plugs 34 are each an electrical quarterwave length at the frequency at which the projecting ends of the rods 28 are decoupled in the driven or power element.

The array also includes a director element 40 in which all parts are similar to the parts comprising the reflector element with the exception that the various overalldimensions are about 5% shorter than the corresponding dimensions of the driven element and except that the electrical length of the internally folded portion of the coaxial line is maintained at the said electrical quarter wave length. Consequently,the parts thereof have been given the same numbers as in the reflector element with the addition of the exponent a.

In installation, the driven element, the reflector el ment and the'director element aremountedin side by side relation with the driven element disposed between the reflector element and the director element. Preferably the distance between the axial lines of the reflector element and the driven element and between the director and driven elements shall not exceed one-quarter of the wave length of the highest frequency for which the array is calculated. Between the'axial lines of the driven element and the director element should be approximately two-thirds the distance between the driven element and the reflector element. 7 V

The operation of the array is the same as in all similarly arranged single frequency arrays with. the added advantage of being resonant at two or more frequencies with the production of useful patterns.

It is believed to be obvious from knowledge of the prior art with respect to single frequency radiators that the above described principles of multi-frequency construction of radiators will apply equally well to parasitic arrays of such radiators as shown in FIG. 11 employing one-half of the radiators illustrated and erecting them vertically with reference to a ground plane keeping in mind the fact that the lateral spacing should not exceed four times a quarter wave length. Also, radiators such as shown in FIG. 1 may be employed in driven arrays in which a plurality of such radiatorsof identical form are connected by a common transmission line to the transmitting and receiving apparatus and they may be arranged in any desired spacing and configuration so long as they are all contained within a circle of a diameter not to exceed sixteen times a quarter wave length at the highest frequency at which said radiators are resonant.

In FIG. 12, there is shown an antenna such as disclosed in FIG. associated or disposed at the focalpoint of a parabolic reflector 41. This embodiment of the invention is limited to those higher frequencies whose wave lengths are such that an antenna of this character may be mounted in a parabolic reflector and would be used only on the ultra high and micro wave frequencies. On those frequencies, the small size of the radiating element makes the positioning thereof at the reflector focal point a practical possibility. Since, except for size, the component parts are the same as shown as in FIG. 10, the same numbers have been applied thereto with the addition of the exponent b. The results of this combination with respect to the directive propagation of signal are the same as heretofore obtained by the use of parabolic reflectors with the added capability of sending and receiving signals on a plurality of discrete wave lengths derived from the antenna construction of the present invention.

The above described embodiments of the invention have been described with particular respect to their capability of sending and receiving signals on a plurality of discrete wave lengths which are harmonically related. This derives from the example given of an embodiment of the in vention designed specifically for use in connection with amateur short wave radio bands which by international treaty are so assigned. However, by way of example, it is found that specific antenna is also resonant at frequencies which are of no concern to the amateur, viz., bands of 11.4 to 11.9 me. and 19.4 to 19.8 me. These frequencies are non-harmonic with respect to the amateur or so-called ham bands. The resonance of the described antenna at these additional frequencies is the result of the above described construction and is calculable by the application of known transmission line theory to the novel coaxial construction. tennas for other frequencies, not necessarily harmonic, can readily be calculated by those skilled in the art in the light of the dis closed novel principles of construction through the application of known formulas on electro-magnetic theory, it being known that by the law of reciprocity that an antenna will receive signals on any frequency at which it is resonant in sending such signals. With these considerations in mind, it is appreciated that in the light of the foregoing disclosure, many variations and modifications will suggest themselves to those skilled in the art and therefore, the invention is not to be deemed to be limited to the exact forms hereinbefo-re disclosed by way of example, but to include as well all such changes and modifications in the parts and in the construction, combination and arrangement of parts as shall come within the purview of the appended claims.

I claim:

1. An antenna for sending and receiving radio signals comprising a conductive base member which is a feed point for radio sending and receiving apparatus, and a rigid internally folded, shorted coaxial transmission line carried by said base, said coaxial line comprising an inner conductive rod having a proximal endconnected to said base and a distal end remote from said base and extending out of said line, and a coaxially disposed conductor means including a first tubular conductor coaxially spaced from and surrounding said rod and extending from a proximal endspaced from said base to a distal end disposed at a lesser distance from said base than the distal end of said rod, an annular, laterally projecting conductive flange carried by said first tubular conductor adjacent said proximal end thereof, a second tubular conductor coaxially surrounding and spaced from said first tubular conductor and having one end thereof conductivcly attached to said flange and having the opposite end thereof extending beyond said proximal end of said first tubular member and attached to said base member.

2. An antenna for sending and receiving radio signals adapted for connection to radio transmitting and receiving apparatus and for operation as a vertical radiator operating in connection with a conductive plane such as the .earth plane comprising a conductive base member which is a feed point for radio sending and receiving apparatus, and a rigid internally folded, shorted coaxial transmission line carried by said base and projecting therefrom, said coaxial line comprising a conductive rod having a proximal end connected to said base and a distal end remote from said base, and a coaxially disposed conductor means also mounted on said base and including a first tubular conductor coaxially spaced from and surrounding said rod and extending from a proximal end spaced from said base to a distal end disposed at a lesser distance from said base than the distal end of said rod, an annular, laterally projecting conductive flange carried by said first tubular member adjacent said proximal end thereof, a second conductive member coaxially surrounding and spaced from said first tubular member and having one end thereof conductively attached to said flange and extending from said flange to said base and attached to said base member with resultant support of said first tubular member with the proximal end thereof spaced from said base member.

3. An antenna for sending and receiving radio signals adapted for connection to radio transmitting and receiving apparatus and for operation as a vertical radiator operating in connection with a conductive plane such as the earth plane comprising a conductive base member which is a feed point for radio sending and receiving apparatus, and a rigid internally folded, shorted coaxial transmission line carried by said base and projecting therefrom, said coaxial line comprising a conductive rod having a proximal end connected to said base and a distal end remote from said base, and a coaxially disposed conductor means also mounted on said base and including a first tubular conductor coaxially spaced from and surrounding said rod and extending from a proximal end spaced from said base to a distal end disposed at a lesser distance from said base than the distal end of said rod, an annular, laterally projecting conductive flange carried by said first tubular member adjacent said proximal end thereof, a second conductive member coaxially surrounding and spaced from said first tubular member and having one end thereof conductivcly attached to said flange and extending from said flange to said base member and attached to said base member with resultant support of said first tubular member with the proximal end thereof spaced from said base member; said first tubular member further including an external coaxial line at the distal end thereof.

4. An antenna for sending and receiving radio signals over any one of a plurality of separate frequencies and disposed remote from a conductive plane; said antenna comprising a pair of identical units arranged in opposed end to end relation along a common axial line above a conductive plane; each of said units comprising a conductive base member which is a feed point, a conductive rod having a proximal end thereof fixed to said base member and projecting therefrom along said axial line to a distal end remote from said base, a tubular conductive member coaxially surrounding and spaced from said rod said rod, conductive means comprising an internally folded, shorted, end fixed to said ,base' member and supporting said'tubnlar'member on said base member in coaxial relation to said rod and with the proximal end of said tubular member spaced from said base member; the base'members of each of said units being positioned in proximity to each other and being'adaptecl' for common connection to radio transmitting and receiving apparatus.

5. The combination with an antenna as claimed in claim '4 of a parabolic radio wave reflector surface arranged with the focal point thereof substantially coincident with the mid lengthof the axis of the antenna.

6. In an antenna for sending and receiving radio signals and comprising a coaxial line havinga tubular outer conductir e member and an inner conductive member disposed within and coaxially spaced from the inner walls of said tubular member, said inner conductive member and said outer conductive member being connected together at one end to form a feed point for the antenna, said inner conductive member extending outwardly of said outer member, said inner member and outer member being radiating elements and said'inner member being maintained in said coaxial relation to said tubular member comprising rigid, porous, non-conductive material engaging the outer surface of said inner member and the inner wall surface of said tubular member; said material having a dielectric constant which, with reference to air, is not greater than 1.1:1.

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